A motor with electronic (stop-and-reverse) braking typically exhibits a three-phase response to collisions. Only in cases of soft collisions will such systems prevent component damage.
Select figure to enlarge.

Topics of discussion:
• Electronic limiters
• Mechanical limiters
• Braking torque
• Collision torque

Electronic overload protection has long been employed on industrial electric motors. In some cases, this internal protection may serve as a viable torque limiting mechanism. Many applications, however, require something more: the protection of a conventional mechanical torque limiter. Anything less will leave machinery vulnerable to overload damage and downtime. Choosing between electronic and mechanical torque limiting requires an understanding of both types of protection and where each fits best.

Electronic devices typically fall into two categories: sensors and controls. Sensors track one aspect of a drive function and provide an output when that function deviates from pre-set norms for a pre-set time period. Controls, on the other hand, continuously monitor machine functions. For example, the difference between the theoretical and actual position of various components might be compared. When either of these systems detect a problem, corrective action can be as simple as shutting down the drive to engaging a brake, or as complex as stopping and reversing the motor.

Originally electronic overload protection only prevented thermal damage. Today it is possible to monitor parameters ranging from current/ voltage, force/torque, rotational frequency, position, temperature, and pressure. Also, with lighter and more powerful motors, more sophisticated controls, and other improvements in drive systems, corrective measures initiated (based on these inputs) are made more rapidly.

The rate at which collision torque increases is critical in determining whether mechanical or electronic limiters provide the best possible protection.
Select figure to enlarge.

Conventional mechanical torque limiters, by contrast, completely disconnect drive and driven components when an overload occurs. The most common types are shear pins, slip clutches, and balldetent torque limiters.

Other types of mechanical torque-limiting devices rely on springs (with special negative rate characteristics) that work in unison with a torque transmission system of balls interfacing two indents. In this setup, preset torque is kept within an acceptable setting tolerance and ensures that even under highly dynamic drive conditions, the clutch disengages during overload.

The design of mechanical torque limiters has also improved significantly over the years. From the basic shear-pin or slip-clutch to state-of-the-art ball-detent clutches that provide backlash free torque transmission, internal splines or keyways subject to fretting and premature wear are eliminated. The newest mechanical torque limiters also utilize special springs with negative spring rates, where increased deflection produces less end force. This eliminates false trips and “breathing”, extending service life while providing much improved accuracy and repeatability.

An example

A motor control scheme designed to stop and reverse the drive when a collision occurs typically exhibits a three phases response before the machine comes to rest, during which

M_b = M_k - M_a

Where M_b = braking torque

M_k = collision torque

M_a = drive torque

In phase one, the drive is still in its normal operating mode, with M_a less than +M_max (where +M_maxis positive maximum drive torque.) As collision torque increases, controls compensate for this load by increasing drive torque in the collision direction. With M_k equal to M_a, the resulting braking torque M_b1 equals zero.

Continue on page 2